I have been looking for information on how rotary valves are sealed, but have not had much luck. Coates has engines with them, and they are oil less to boot or so they claim, but how do you get them to seal during a compression stroke?

The only thing I can think of is very tight tolerances but this can lead to carbon buildup in the very small gap between the rotary valve and the rim around the hole in the top of the cylinder.

dual overhead rotary valves

thank you @Mauro that is similar to a design I devised up for a project I will be doing, rotary valve suzuki samurai / geo metro / swift / etc aftermarket heads. I was planning on borrowing the spring loaded ceramic seals a wankel uses to seal the rotor but the spring and overall design would catch quite a bit of carbon buildup. I am wondering how they get their seals to seal without the carbon buildup or springs? or are they letting the carbon be the seal? Or if the clearances were tight enough would it matter?

  • Why did this get a negative?
    – Cc Dd
    Nov 17, 2016 at 5:36
  • or rather a downvote
    – Cc Dd
    Nov 17, 2016 at 5:45
  • 5
    Not sure, it doesn't make any sense. +1 It's a good question. Nov 17, 2016 at 6:16
  • I'd love to hear more about your plans for the aftermarket heads – especially about the manufacturing and design process.
    – dlu
    Nov 19, 2016 at 15:45

4 Answers 4


Diagram of valves From the same site, an explanation of the differences between the two types of valves shows theres still a valve seal. I suspect this is the only part where clearences have to be tight.

  • good pic I updated my question with something a bit more specific.
    – Cc Dd
    Nov 17, 2016 at 20:30

Since there really isn't any information out there directly related to your question, we can only speculate with an informed or educated understanding of what's going on.

To that end, on the Coates webpage, they make a few statements about how the spherical valve works inside the head. They state:

The Coates Spherical Rotary Valve comprises two spherical rotary valves assembled on two separate shafts - one for inlet and one for exhaust. They rotate on ceramic carbon bearing with no oil lubrication, the spheres do not make contact with any part of the housing. The seals are a floating type and are also made of a ceramic material. They have two piston rings and are floating in a small cylinder-type chamber, they are activated by the compression and the combustion strokes of the engine which allows 100 percent sealing effectiveness, when compressed.

Maybe you get this, but I'll point it out anyway, the seal at the spherical valve is not two pieces, but one. I don't think the animated gif on their website does this any justice. In the blown-up/cut-away image below, the exhaust side I think is more representative of what's going on, but doesn't really clearly show it:

enter image description here

In the image, it sort of looks like there are actually two pieces of the seal on one valve which keeps the valve sealed, when in actuality there is only one. It is round (or cylindrical) in shape. It has a bevelled edge (as you can see in the image) which the spherical shape of the valve rides against. Hopefully this simple drawing might help show what I'm talking about:

![enter image description here

In my drawing, the red, circular piece at the bottom is the seal and the gray (or off-white) portion is the valve. As the sphere of the valve rotates, the beveled edge of the seal is pushed up against it to create the physical seal. During the compression and power strokes, the seal is forced up against the valve with "100% sealing effectiveness". The cylindrical seal is ceramic and the sphere of the seal is highly polished. As the valve rotates, it is continually cleaned by the seal, ensuring no carbon deposits will interfere with the operation of the valve itself. Since there is 100% sealing, no hydrocarbons can escape past the seals to clog the valve itself. Everything is maintained within the cylinder or through the port as the intake/exhaust portion is happening. Also remember, as long as the combustion process is complete, there shouldn't be very minimal carbon buildup.

(Side note: A simple water injection in the intake path would keep everything free of any possible carbon buildup.)

The other part which ensures there aren't any carbon deposits is that there isn't any oil in the head. The valve rod (I don't know what else to call it ... the rod which the spherical valves reside on rotates the valves) rotates on ceramic carbon bearings with no oil lubrication. Due to this lack of oil, there's also a lack of carbon and sludge buildup you'd see with regular poppet valve engines. There's no possibility of oil slipping past valve seals to clog things up. This means it stays cleaner than its poppet valve counterpart.

  • Maybe, valve shaft?
    – dlu
    Nov 19, 2016 at 15:18
  • @dlu - That would work as well! Nov 19, 2016 at 15:23
  • 1
    I wonder how the carbon seals hold up in a high RPM setting, say 25k-50k? The thing that makes these valves really neat is the fact you can spin engines up to really high RPMs since you have 0 valve float hardly any parasitic drag on what used to be cams which meant the only things holding you back are airflow and pistons deciding to leave stage left, usually in a violent fit of rage.
    – Cc Dd
    Nov 19, 2016 at 19:10
  • These valves would still only be operating at 1/2 engine speed ... you planning on revving a piston engine up to 50-100k rpm? That might be a neat trick without the "violent fit of rage" you talk about :o) I'd suspect they'd work just fine, but that again would pretty much be a guess. Nov 19, 2016 at 19:13
  • @Pᴀᴜʟsᴛᴇʀ2 very small motions think of a F1 style engine, 30 piston 1 liter w block and not a 2 liter vtwin or something that has 2 stories worth of stroke lh6.ggpht.com/-x2JMebj3TDk/UUsbStYorpI/AAAAAAAAmes/MhA2z_ZDDcg/… I will admit they are not the tallest stories out there but still a good height for a mobile home.
    – Cc Dd
    Nov 19, 2016 at 19:21

It seems like the downside of this design is that it would be much harder to modify valve timing. I would think that compared to this design, grinding a new camshaft would be a relatively simple process – and easier to think about as well.

  • In the rotary shaft you can put servos that can twist each rotor individually. these can be controlled wirelessly and powered by a positive and negative trace around the rotor shaft. 2 brushes would contact this and there would be a small battery for each since you don't want them powering on and off due to brush wear. in short harder yes impossible no.
    – Cc Dd
    Nov 19, 2016 at 17:28
  • -1 @dlu I respect you and you provide a lot of great insights on this site. But, this particular answer does not answer any of the questions by OP. Sorry. Friends still? Nov 19, 2016 at 17:45
  • @ZachMierzejewski, sure. Your point is well taken. I was thinking about the valves and why a cool seeming idea didn't seem to be getting much update and lost track of what the question was. Maybe I'll leave it up for a bit and see if it attracts a few more down votes so I can get the Disciplined when I delete it :-)
    – dlu
    Nov 19, 2016 at 17:58
  • @dlu I am going to upvote it because it brings up a decent point but you should be more specific that you are talking about a regrind rather than other forms of variable valve timing. a regrind would not be necessary as there is no wear as experienced on a regular cam. but variable valve timing could be a little trickier however it also took us a loooong time to develope our current variable valve timing schemes.
    – Cc Dd
    Nov 19, 2016 at 18:28

At a guess I would likely say ptfe since it’s capable of both operating at the pressure and temperature required. Plus since we have already been using it for similar applications it would be the easiest way to engineer such a cam.

Below is a diagram of how this is applied to ball valves. enter image description here

  • Ignore the pressure and temperature ratings as they can be modified by both thickness and material makeup. Apr 8, 2021 at 11:38

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